The best current methods of reproducing images of halftones pulse-width-modulate the digital video signal used to drive the raster print engine that reproduces the image. The image may be reproduced using either halftone dots or vertical lines of varying width to create the gray scale.
U.S. Pat. No. 4,800,442 discloses a line screen apparatus in which the raster print engine drive signal is pulse-width-modulated by comparison of an analog video data signal to a sawtooth wave form. The analog signals are derived from lookup tables addressed by the digital input signals, typically 600.times.600 dpi bitmaps. Another example of such a pulse-width-modulation technique is disclosed in U.S. Pat. No. 3,916,096.
There are several problems with these pulse width modulation techniques which have prevented them from creating high quality image reproductions. Typically, the sawtooth pattern has the same period as the pixels. An analog video signal, having a discrete value in each pixel time period, is then compared to the sawtooth for generating the pulse-width-modulated signal. However, because the sawtooth is symmetric and the analog signal is single-valued, the digital video signal pulses are of necessity centered within each pixel, creating printed lines or dots which are also of necessity centered in the pixels. These pulse-width modulation print engine drivers thus can not alter the position of the printed line segment within the pixel area nor can they alter the center of gravity of the part of the image reproduced in a pixel. Accordingly, such systems do not have the flexibility to reproduce with great precision either halftone images or continuous-tone text and line art images.
Another problem which greatly affects the image quality of these systems is their method of establishing line-to-line synchronization of the video signal supplied to the raster scanning print engine, critical for high quality images. Horizontal synchronization is accomplished in U.S. Pat. No. 4,800,442 by comparing a seventy-two MHz reference clock signal to the horizontal sync signal generated by the laser printer beam detector to enable generation of pixel clock timing signals at a frequency of 18 MHz. However, this technique results in a line-to-line synchronization error of up to 25% of the pixel clock period, resulting in significant horizontal jitter. In addition, the high frequency reference is a potential source of RFI in the system.
The combination of the symmetric nature of the pulse width modulated digital video signals and the horizontal jitter of the known halftone image reproduction techniques has significantly limited the quality of the reproduced image. As a result, there exists a need for relatively inexpensive, high-quality halftone image reproduction apparatus.